关于collapsing margins详细说明可查看CSS21规范的8.3.1 Collapsing margins。
另外对于一个maring:10px的DIV元素，如果此元素没有内容，并没有设置padding和border值，其上部的margin与其下部的margin也会重合，也就是此DIV在垂直方向上只会占据10px的高度。如果此DIV的top或者bottom与别的元素发生过重合，则其本身的top和bottom margin不会再重合。
规范原文说明如下：
If the top and bottom margins of a box are adjoining, then it is possible for margins to collapse through it. In this case, the position of the element depends on its relationship with the other elements whose margins are being collapsed.
If the element’s margins are collapsed with its parent’s top margin, the top border edge of the box is defined to be the same as the parent’s.
Otherwise, either the element’s parent is not taking part in the margin collapsing, or only the parent’s bottom margin is involved. The position of the element’s top border edge is the same as it would have been if the element had a non-zero top border.